Take a close look at a reproduction 1880 dual gauge A frame gallows turntable at the Great Western Steam Up at the Nevada State Railroad Museum in Carson City.
This type of turntable was in use in the 19th century when locomotives were smaller. Later turntables were steel bridge monsters for turning the massive locomotives of the twentieth century. These early turntables were made of wood and turned by hand!
The weight of the locomotive is carried on a "spider" at the center of the turntable. The ends float requiring that the locomotive be placed in the exact center of the turntable bridge.
We also take a look at an On3 model of a gallows turntable built by Steve Strebel. An excellent scratch-built model it shows the workings of the center "spider".